Pssst — heard the one about the lethal scandal that reaches to the upper levels of the Obama administration and may turn out to be bigger than Watergate? A crime in which two American agents have been killed, and which the Justice Department has been furiously trying to cover up for more than 18 months?

If you answered “no,” you’re not alone. The mainstream media have largely ignored the lethal — and most likely deliberate — “gunwalking” operation known as Fast and Furious. The plan, as ridiculous as it sounds, was to knowingly sell weapons to Mexican drug cartels, in hopes of tracing the guns. Of course, our government lost track of them.

With the president’s startling claim last week of executive privilege on behalf of his embattled attorney general, Eric Holder, the story’s now too big to ignore. So here’s what you need to know:

It’s about innocent victims

One of them is Brian Terry, who died at age 40 on Dec. 15, 2010 in Rio Rico, Ariz., just north of Nogales and about 10 miles from the Mexican border. The night before, Terry and his elite Border Patrol unit had encountered a “rip crew” — a Mexican gang that preys on smugglers and illegal immigrants. In the shootout, Terry was mortally wounded by an AK-47 and died the next day.

That AK-47, along with one other, was traced back to an Arizona gun shop that sold it in the Fast and Furious operation. And that kicked off a scandal that widens by the day.

The Detroit-born Terry, who served in the Marine Corps and as a police officer in Lincoln Park, Mich., before joining the Patrol, had a reputation for fitness, toughness and bravery.

“I do not fear death for I have been close enough to it on enough occasions that it no longer concerns me,” the Detroit native wrote in a poem shortly before his death. “But I do fear the loss of my honor, and would rather die fighting than have it said I was without courage.”

Terry’s family has since sued the ATF for $25 million, citing Fast and Furious as the cause.

But it’s not just Terry

Two months later, Immigration and Customs Enforcement agent Jaime Zapata, on assignment in Mexico, was murdered by drug cartel members, who also wounded his partner. The gun that killed him, an AK-47 knockoff, was traced back to a gun shop in Dallas.

Last week, Zapata’s family filed suits against the ATF, ICE and the FBI totaling $50 million.

The two agents’ death come in addition to the deaths of hundreds of Mexicans killed with F&F weapons in their country’s lethal drug wars.

This isn’t about principle — it’s about protecting the AG from the consequences of a self-inflicted scandal with the potential to bring down the president.

Which is why, for a year and a half, Holder and his underlings have lied, stonewalled, weaseled, misdirected, finger-pointed, flip-flopped and taken the Fifth in a desperate attempt to wriggle free of the House Oversight Committee, led by Rep. Darrell Issa (R-Calif.) and the Senate Judiciary Committee, whose ranking member is Iowa Sen. Charles Grassley.

Issa and Grassley are trying to find out which political appointee or elected official green-lighted the lethally ill-conceived project, under which some 2,000 high-powered weapons from American gun shops in the southwest were funneled to drug cartels in Mexico.

Astoundingly, the program was “supervised” by the US Attorney’s office in Arizona and implemented by the Bureau of Alcohol, Tobacco, Firearms and Explosives (both subdivisions of the Justice Department) — if “supervised” is the right word.

Congress has been investigating pretty much ever since, calling witnesses, subpoenaing hundreds of thousands of documents – and getting as little cooperation from Holder as possible.

Bush didn’t do it

Fast and Furious bears a superficial resemblance to a much smaller Bush-era operation known as Operation Wide Receiver, in which some 450 guns walked into Mexico. But that operation was conducted with the full knowledge of Mexican authorities — unlike Fast and Furious, which was kept secret — and the government at least attempted to implant radio tracking devices on the weapons. When that failed, the operation was terminated.

The scandal will now widen

By embracing his doomed attorney general, Obama has now forfeited plausible deniability and tied Fast and Furious directly to the White House, a decision he’s likely to regret — especially since he continues to assert that nobody in the West Wing were aware of the operation. Indeed, back in March 2011, Obama claimed he’d first learned of Fast and Furious “on the news” and said that Holder knew nothing.

Yeah, right. A document dump in early December last year proved conclusively that Holder’s chain-of-command subordinates knew all about it. Indeed, that knowledge now lies at the center of the contempt citation and Obama’s executive privilege claim — and of the mystery: how high does this go?

Holder got slapped with a contempt charge by the House Oversight Committee owing to his refusal to turn over a tranche of subpoenaed documents relating to the now-notorious letter of Feb. 4, 2011, in which Justice categorically denied any knowledge of, or involvement with, Fast and Furious. This despite hair-raising whistleblower stories from brave ATF agents describing their feelings of helplessness as batches of weapons were transferred from the US to Mexico right under their noses — and being ordered not to interfere.

But that letter, by assistant attorney general Ronald Weich — who conveniently announced his sudden retirement earlier this month — was “withdrawn” last December. “Facts have come to light during the course of this investigation that indicate the Feb. 4 letter contains inaccuracies,” wrote a Justice bigwig, wiping the egg from his face.

Obama’s executive privilege declaration covers the documents that directly pertain to the aftermath of the Feb. 4 letter, as Justice internally debated — according to Holder — “how to respond to congressional and related media inquiries into that operation.”

How to deal with potential perjury or obstruction charges is more like it.

It’s really about stealth gun control

Even before Obama was inaugurated, gun control was high on his wish list, including the restoration of the Clinton-era ban on “assault weapons.” So the most plausible explanation for the fine mess the administration currently finds itself in is this:

Wishing to “prove” the lie that 90% of the guns used in Mexican drug violence originate in America (the real figure is closer to 17%), Justice used the failed Operation Wide Receiver as the model for a larger operation deliberately designed to fail.

That way, they could point in feigned horror at the recovered American weapons and crack down on legitimate gun dealers — the very dealers they had forced to sell weapons to the cartels via “straw purchasers” in the first place.

In short, the truth is that Fast and Furious was most likely a murderously cynical assault on the Second Amendment — and one whose multiple ghosts will now haunt the Obama administration’s remaining days.

Michael Walsh is a Post columnist and the author of “The People v. the Democratic Party,” out next month.